Currently, in the field of clinical diagnostic testing, it is required to measure a wide variety of substances that serve as indicators of disease diagnosis for a large number of specimens in a short time and with high accuracy, and to feed back the results quickly and accurately to the treatment site. For example, accurate quantification of a trace amount of an analyte has been carried out many times by an analytical system in which physiologically active substances (proteins such as antibodies, enzymes, receptors, or the like; antigens; nucleic acids such as DNA, RNA, or the like; sugar chains; or the like) are bound to the particle surfaces, such as an immunological measurement utilizing an antigen-antibody reaction. In particular, as a method for improving the detection sensitivity and accuracy, a latex agglutination method utilizing particles in which an antigen or antibody against a substance to be analyzed is carried on the surface of synthetic polymer (so-called latex) particles, such as polystyrene, is known. The latex agglutination method is a method in which a substance to be analyzed is measured in a short time by visually or optically detecting the degree of agglutination of latex particles caused by a reaction of the substance to be analyzed with an antigen or antibody bound to latex particles.
A sandwich method, in which the latex particles are made of magnetic particles, and a substance to be analyzed is trapped with the first antibody bound to the magnetic particles, and an unreacted substance and the like is washed while being accumulated using a magnet (so-called B/F separation), and analysis is carried out by adding the second antibody labelled with a substance capable of generating a signal, such as an enzyme, a fluorescent agent, or the like, is also frequently used.
Many substances to be analyzed, which are quantified by an antigen-antibody reaction, are generally trace components contained in a biological sample, and it emphasizes the quantitative performance in the low-concentration region. However, since the concentration of the substance to be measured sometimes shows a high value abnormally, depending on the degree of disease progression, there is a demand for a reagent capable of accurately measuring from a low value to a high value.
When latex particles are used in the latex agglutination method or the sandwich method, it is necessary to immobilize an antigen or antibody on the surface of the latex particles. As the immobilizing method, a method in which the latex particles are physically or chemically carried is used. For example, a method in which an antigen or antibody is directly immobilized on the latex particles by physical adsorption, or a method in which an antigen or antibody is bonded to the surface of the latex particles, by a covalent bond via a functional group, such as a carboxyl group, a maleimide group, an amino group, a mercapto group, a hydroxyl group, an aldehyde group, an epoxy group, or the like, is used.
In comparison with the physically binding method, the chemically binding method is superior, because the antibody is bonded on the surface of the latex particles via a covalent bond, and can be stably carried on the surface. Various methods of bonding a functional group to the surface of latex particles have been reported, but there is a problem that the control of a variation in production lot and the amount of the bonded functional group are difficult, and particles that stably and accurately carry a physiologically active substance cannot be prepared (Patent literatures 1 and 2).
Further, various methods for quantifying the amount of functional groups on the surface of latex particles have been reported, and, for example, in a method wherein acid-base titration, which is used for the quantification of a carboxyl group, is performed, and quantification is carried out from a change in electrical conductivity, when the amount of the functional group is extremely small, accurate quantification is difficult. Since the latex particles used in clinical diagnosis are small, and the amount of the functional group becomes small, and thus, accurate quantification becomes difficult, and it is a problem that stable supply to produce clinical diagnostic reagents cannot be done.
Further, it is important for an application to biomaterials, such as clinical diagnostic agents or the like, to control the distance between the latex particle surface and the functional group for carrying a physiologically active substance, for binding a biological sample. Conventionally, there is a technique wherein a compound having double bonds, and further having functional groups via a hydrophilic polymer such as polyethylene glycol, as a macromonomer, is copolymerized with a monomer such as styrene, to bond on the latex surface. However, there is a problem that it is difficult to control the amount of the bound functional group, and to obtain latex particles having small production lot to lot difference.
Further, as a problem of the latex agglutination method, there may be mentioned a non-specific reaction, in which non-specific agglutination occurs by adsorbing contaminants other than the substance to be measured on the surface of the latex particles (non-specific adsorption), and as a result, this non-specific agglutination is quantified as the substance to be measured.
As a general method of inhibiting a non-specific reaction, there may be mentioned a method in which bovine serum albumin (BSA) or sugar is previously adsorbed to latex particles. However, it is difficult to uniform the adsorbed amount of BSA or sugar, or lot to lot difference of BSA, which is a protein derived from an organism, and it is difficult to stably supply an analytical reagent by a latex agglutination method (Patent literature 3).
Further, a method in which an antigen or antibody against the substance to be analyzed is present at the time of synthesis of polymer particles, and the polymer particles are modified with the antigen or antibody, to inhibit the non-specific adsorption, which previously occurred, has been reported, but it is not sufficient (Patent literature 4).
As a method of inhibiting non-specific adsorption, a method in which a compound having a polymerizable double bond at the end of polyethylene glycol, which is known to be effective for inhibition of a non-specific reaction, is used as a monomer, to synthesize polymer particles, is known, but it is not sufficient (Patent literature 5).
A technique of inhibiting non-specific adsorption wherein a monomer effective for inhibition of non-specific adsorption is allowed to be present during polymerization, but it is not sufficient (Patent literature 6).
Recently, a method of inhibiting a non-specific reaction by adsorbing a block master (JSR Corporation), in which a hydrophobic unit is incorporated into polyethylene glycol, to the surface of latex particles is developed. However, there are problems that lot difference increases due to the complication of a preparation step of an immunological analytic reagent, and that it is difficult to quantify the modified amount of the block master.
Therefore, latex particles used in clinical diagnostic reagents are required to improve the stability of the manufacture of clinical diagnostic reagents and the accuracy of clinical diagnostic reagents, but it has not been achieved yet.